1. Field of the Invention
The present invention relates a developing agent carrier for carrying and conveying a developing agent used to develop an electrostatic latent image formed on an image carrier by using an electrophotographic scheme or electrostatic recording process, and a developing unit and image forming apparatus which include such developing agent carriers.
2. Related Background Art
In a conventional image forming apparatus, such as a copying machine or printer, the surface of a drum-like electrophotographic photosensitive member, i.e., a photosensitive drum, is uniformly charged, and an electrostatic latent image is formed on the surface by performing exposure in accordance with an image signal. This electrostatic latent image is developed into a toner image by a developing unit, and the toner image is transferred onto a recording medium. The image is then fixed to obtain a permanent image. As such an apparatus, an image forming apparatus having a developing unit for developing an electrostatic latent image on an image carrier by using an insulating, nonmagnetic, single-component toner is known.
FIG. 4 shows a developing device used in a conventional image forming apparatus.
A developing unit 4 has a developing agent vessel 15 open at a position to oppose a photosensitive drum 1. A developing roller 11 as a developing agent carrier is partly exposed outside the developing agent vessel and supported in the developing agent vessel 15 to be rotatable in the direction indicated by an arrow Y.
The developing agent vessel 15 contains a nonmagnetic single-component developing agent (to be simply referred to as toner hereinafter) as an insulating developing agent. An agitating member 13 agitates this toner and conveys it to an area where the developing roller 11 is in contact with a toner feed roller 12 as a means for feeding the toner to the developing roller 11.
The toner comes into contact with the developing roller 11 and is fed onto the developing roller 11 by the reflection force generated when the toner is fictionally charged in the slidable contact area between the developing roller 11 and the toner feed roller 12 that rotates in the direction indicated by an arrow Z, i.e., a direction opposite the rotational direction Y of the developing roller 11. The thickness of the toner fed onto the developing roller 11 by the toner feed roller 12 is controlled by a developing blade 14 as a means for controlling the thickness and charge amount of toner upon rotation of the developing roller 11 in the direction indicated by the arrow Y. This toner is applied to the surface of the developing roller 11 and carried/conveyed on the developing roller 11 to reach the developing area where the photosensitive drum 1 rotating in the direction indicated by an arrow X is in contact with the developing roller 11, i.e., a developing nip N.
A developing bias is applied from a power supply (not shown) to the developing roller 11. The toner is electrostatically attracted to the electrostatic latent image on the photosensitive drum 1 to form a toner image. The toner that does not contribute to the development of the electrostatic latent image and is left on the surface of the developing roller 11 reaches the developing agent vessel 15 upon rotation of the developing roller 11. This toner is scraped off from the surface of the developing roller 11 and recovered into the developing agent vessel 15.
The developing roller 11 that comes into contact with the rigid photosensitive drum 1 as an image carrier to perform development is preferably a roller having an body. In the deveoloping unit 4 including the developing roller 11 having an elastic member, a metal developing blade 14 is suitably used as the developing blade 14 to charge nonmagnetic single-component toner.
The above conventional image forming apparatus includes the developing unit using an insulating nonmagnetic single-component developing agent as a developing agent. The toner is fed from the toner feed roller 12 to the developing roller 11 by the reflection force generated when the toner is rub/charged in the slidable contact area between the toner feed roller 12 and the developing roller 11. Therefore, the toner itself is nonmagnetic, and hence this developing device is advantageous in forming a color image. In a developing unit using a magnetic single-component developing agent as a developing agent, as is known, a member that generate a magnetic field, i.e., a magnet, is disposed in the developing roller 11 to apply a magnetic force so as to feed and convey the toner. However, the arrangement of this image forming apparatus is basically the same as that of the above apparatus.
In the conventional image forming apparatus, which is designed to develop an electrostatic latent image on the photosensitive drum 1 by bringing the developing roller 11 having the elastic member into contact with the photosensitive drum 1, i.e., include a contact developing step, the surface of the developing roller 11 vibrates owing to the friction between the developing roller 11 and the photosensitive drum 1 in the developing nip N (developing area) where the developing roller 11 comes into contact with the photosensitive drum 1. As a result, in developing operation, the toner image on the photosensitive drum 1 is scattered in the developing nip N.
If the surface of the developing roller 11 is greatly distorted when the developing roller 11 comes into slidable contact with the photosensitive drum 1, the vibrations of the surface of the developing roller 11 are accelerated, and the developing roller 11 itself vibrates. As a result, the developing nip N varies in width. In the worst case, the developing roller 11 itself greatly vibrates, the developing roller 11 and photosensitive drum 1 are set in a noncontact state periodically. As a consequence, a non-development area periodically appears on the developing roller 11 in the developing unit to which a DC developing bias voltage is applied, resulting in pitch irregularity on the image formed a recording medium.
The following is the reason why a distorting force and vibration are generated on the surface of the developing roller 11 when the developing roller 11 comes into slidable contact with the photosensitive drum 1 in this manner. Conventionally, the developing roller 11 is an elastic roller having an elastic member including a solid layer made of silicone rubber, EPDM, or urethane rubber or sponge layer, and the dynamic friction coefficient of the surface of the developing roller 11 is generally set to about 0.2 to 1.5. As the coefficient of dynamic friction of the developing roller 11 increases, the frictional force between the developing roller 11 and the photosensitive drum 1 increases. As a result, the surface of the developing roller 11 vibrates. The vibration of the surface of the developing roller 11 becomes noticeable as the hardness of the developing roller 11 decreases. As a consequence, the toner is scattered on an output image on a recording medium.
Assume that a solid layer or sponge layer having a low hardness is used as an elastic layer, the dynamic friction coefficient of the developing roller 11 exceeds 0.2, and the hardness measured by an Asker C durometer is about 40 or less. In this case, in particular, the developing nip N between the developing roller 11 and the photosensitive drum 1 increases to increase the contact force on the surface of the developing roller 11 with respect to the surface of the photosensitive drum 1. As a result, the frictional force between the developing roller 11 and the photosensitive drum 1 increases. If, therefore, the dynamic friction coefficient of the surface of the developing roller 11 is large, the distortion of the surface of the developing roller 11 is large.
The xe2x80x9cdynamic friction coefficientxe2x80x9dused in this specification is defined as the value measured by the following device and method.
FIG. 5 shows the schematic arrangement of a device for measuring a dynamic friction coefficient. One end of a 0.03-mm thick thin stainless steel plate 24 is set on a digital force gauge 21, and a weight 22 is added to the other end portion of the steel plate 24. In addition, the developing roller 11 comes into contact with the middle of the stainless steel 24 such that an angle xcex8 defined by the two ends of the stainless steel plate 24 becomes 45xc2x0. The digital force gauge 21 is adjusted to 0 in advance without the weight 22 and stainless steel 24.
After the digital force gauge 21 is stabilized, the developing roller 11 is rotated in the direction indicated by an arrow R. The frictional force between the developing roller 11 and the thin stainless steel plate 24 is measured by the digital force gauge 21 at this time. A measurement value is obtained as follows. The analog value output from the digital force gauge 21 is sampled by a recorder at a frequency of 10 Hz, and the sample data is calculated by a computer according to the following equation
xcexc=(1/xcex8)ln(F/W)
and, the calculation values corresponding to one revolution of the developing roller 11 are averaged.
In this case, the value of xcexc is the dynamic friction coefficient in this specification, W is the sum of a weight W1 of the weight 22 and a weight W2 of the thin stainless steel plate 24, and F is the measurement value obtained by the digital force gauge 21.
The dynamic friction coefficient of the surface of the developing roller 11 with respect to the thin stainless steel plate 24 is measured by the device for measuring the above dynamic friction coefficient for the following reason. In the conventional developing unit, a thin stainless steel plate having a thickness of about 0.1 mm is generally used as the developing blade 14. In addition, as the photosensitive drum 1, a member having a photosensitive layer having a thickness of about 10 xcexcm formed on an aluminum plate or the like is used. For this reason, as the dynamic friction coefficient of the surface of the developing roller 11, the value obtained with respect to the thin stainless steel plate 24 is preferably used and compared. The resultant value seems to reflect the present situation.
The above problem, i.e., the problem that the distortion of the surface of the developing roller 11 becomes large as the dynamic friction coefficient of the surface of the developing roller 11 increases, can be suppressed by reducing a biting amount S of the developing roller 11 into the photosensitive drum 1, i.e., reducing the contact pressure between the developing roller 11 and the photosensitive drum 1. A reduction in contact pressure, however, increases the probability of contact failure between the developing roller 11 and the photosensitive drum 1. As a result, a development failure tends to occur.
Conventionally, therefore, the hardness of the developing roller 11 is increased to reduce the distortion of the surface of the developing roller 11 even if the dynamic friction coefficient of the surface of the developing roller 11 is large, thereby suppressing vibrations and preventing the toner from being scattered as in the above case. More specifically, the developing roller 11 having a solid layer with a hardness of 40 to 45 (JIS-A) (about 50 to 55 with an Asker C durometer) is generally used.
If, however, the hardness of the developing roller 11 increases, the contact pressure between the developing roller 11 and the photosensitive drum 1 increases As a result, the driving torque of the developing roller 11 increases. If this driving torque increases, the developing roller 11 rotates irregularly to cause developed state irregularity when a driving motor lacks torque. As a consequence, density irregularity occurs on an output image on a recording medium.
If the hardness of the conventional developing roller 11 having a large dynamic friction coefficient is increased to increase the contact pressure between the developing roller 11 and the photosensitive drum 1, the nonmagnetic single-component toner at the developing nip N is greatly damaged, resulting in a deterioration in toner. As a result, if the toner cannot be replaced alone, the service life of the developing unit or image forming apparatus becomes short. This phenomenon becomes noticeable especially when low-melting toner advantageous for low-temperature fixing is used.
Consider the roughness of the surface of the developing roller 11. In order to set the amount of toner applied to the developing roller 11 to a predetermined value, Rz (JIS 10-point average roughness) must be set to a predetermined value, and Rz is generally set to about 5 to 10 xcexcm.
If, however, Rz is set to a large value, the number of times toner comes into contact with the surface of the developing roller 11 increases to enhance the charging performance. As a result, the toner coat amount on the developing roller 11 increases. In consideration of this phenomenon, an increase in Rz is not necessarily suitable for the developing roller 11 in terms of the above dynamic friction coefficient of the developing roller 11.
As described above, in the conventional image forming apparatus, in consideration of the formation of a toner coat on the developing roller 11, the dynamic friction coefficient of the surface of the developing roller 11 is large, and hence the frictional force at the developing nip N is large. Attempts to decrease the hardness of the developing roller under these circumstances in the prior art will increase the vibration of the surface of the developing roller 11 because of the large frictional force at the developing nip N. For this reason, in the prior art, the hardness of the developing roller 11 is increased to suppress this vibration. If, however, the hardness of the developing roller 11 is increased, the contact force at the developing nip N increases, and the driving torque of the developing roller 11 increases. In addition, the toner is greatly damaged, resulting in a deterioration in toner. As a consequence, low-melting toner, which allows low-temperature fixing and can be a useful material, cannot be properly used. Demands therefore have arisen for means for solving such a contradiction.
It is an object of the present invention to provide a developing agent carrier, developing unit, and image forming apparatus which can stably obtain a high-quality image.
In order to achieve the above object, according to the present invention, there is provided a developing agent carrier comprising an elastic portion which comes into contact with an image carrier, wherein an Asker C hardness of the elastic portion is not less than 20 and not more than 40, and a dynamic friction coefficient of a surface of the elastic portion is not less than 0.01 and not more than 0.2.
In addition, in order to achieve the above object, according to the present invention, there is provided a developing unit comprising a developing agent carrier for carrying a developing agent, the developing agent carrier including an elastic portion which comes into contact with the image carrier, wherein an Asker C hardness of the elastic portion is not less than 20 and not more than 40, and a dynamic friction coefficient of a surface of the elastic portion is not less than 0.01 and not more than 0.2.
Furthermore, in order to achieve the above object, according to the present invention, there is provided an image forming apparatus comprising an image carrier for carrying a latent image, and a developing agent carrier for carrying a developing agent, the developing agent carrier including an elastic portion which is in contact with the image carrier, wherein an Asker C hardness of the elastic portion is not less than 20 and not more than 40, and a dynamic friction coefficient of a surface of the elastic portion is not less than 0.01 and not more than 0.2.
It is another object of the present invention to provide a developing agent carrier, developing unit, and image forming apparatus, in which a developing agent carrier having a low hardness can be driven with a low torque in the image forming apparatus including a contact developing step using a single-component developing agent, the apparatus can be simplified and reduced in size, scattering of toner and pitch irregularity due to the deforming force of the surface of the developing agent carrier can be prevented while a high charging property for the toner is maintained, a deterioration in toner due to a high contact pressure at a developing nip N can be prevented, and a high-quality image can be stably obtained.
The above and other objects, features, and advantages of the present invention will be apparent from the following detailed description in conjunction with the accompanying drawings and the appended claims.